I first encountered ENT in reruns, so I was behind the times. That having been said...
"In a Mirror, Darkly" was the umpteenth attempt by the Berman sub-franchise to capitalize on the "alternative universe"/"alternate reality" trope, and, looking at these stories strictly on their plausibility, I would say that the only one of these that holds up is "Yesterday's Enterprise". None of the others ever made any sense. It's all "let's try tinkering with the sets and costumes and get all the characters to act like Nazis, for a neat-o story idea" gimmick.
From a brainless action-adventure standpoint, at least "In a Mirror, Darkly" was kinda fun. The Gorn thing was obviously unintentionally hilarious. The whole Defiant/"The Tholian Web" take was silly. Kirk & crew found the Defiant derelict in space, with her systems obviously slowly going offline and the ship, in McCoy's words, "dissolving". Defiant was not salvageable. But it was neat to see Scott Bakula and the ENT gang wearing TOS uniforms and trying to learn (waaay too quickly) how to use Federation technology. It was neat to see the (suddenly revived) Defiant kick serious ass.
One serious gaffe in IAMD part 2 was when the Defiant had sustained direct hits which apparently violated her hull when she battled the Avenger; yet after the battle there is no visible damage and it seems like the battle never happened. That was just plain ridiculous.
I absolutely do not buy the whole Tholian scheme to lure a ship from a future they shouldn't know about subplot. That was goofier than TNG's Sela story arc.
Actually, I regard YE as superior to all of these "alternivserse" shows, including the original "Mirror, Mirror". At least YE had some serious plotting and concepts behind it, restructuring the TNG Universe because some strange event altered history, an idea vaguely borrowed from "City on the Edge of Forever". YE took that notion seriously and thoughtfully told a story based on where such a mishap could logically lead. "Mirror, Mirror" and its TNG/DS9/ENT descendants had no such underpinnings, instead wasting the viewer's time by using the "alterniverse" concept to romanticize a dystopic, nazified, perverted revision of Roddenberry's "sunny side up" vision of the future. Silly and tasteless.